The present invention relates to a battery charging system for use in an electric vehicle having at least a high-voltage battery and a low-voltage battery. In particular, the battery charging system is configured to charge the high-voltage battery via an external power supply, and charge the low-voltage battery either from the high-voltage battery or from the external power supply.
Conventionally, in a plug-in hybrid or electric vehicle, various components are supplied by a standard low voltage of 12 or 24 volts (12V/24V, hereinafter called low-voltage battery). This voltage is generated by a separate DC/DC converter connected to a high voltage power source (e.g., a propulsion battery), where the DC/DC converter converts the high voltage into the 12V or 24V low voltage. In some cases, a separate 12V or 24V generator driven from rotating components of the combustion engine is used.
To provide sufficient power for all low voltage supplied functions to operate in critical situations, the typical output power of such a conventional DC/DC converter is up to 4 kW in a 12V system for passenger car applications, and up to 12-15 kW in a 24V system for commercial or off-highway vehicle applications.
As described herein, in order for a vehicle to charge its high-voltage battery from an external power supply, the vehicle typically includes an on-board charger (OBC). The on-board charger may be adapted to charge the high-voltage battery as quickly and/or efficiently as possible, and to utilize the available power from the external power supply (e.g., a public power grid) while adhering to safety regulations regarding connection of equipment to the public power grid system.
When the vehicle is plugged into the public power grid for a period of time longer than necessary to charge the high-voltage battery, the 12V or 24V battery (low-voltage battery) will be depleted because the control logic controlling the on-board charger and other on-board control and safety functions are powered by the low-voltage battery, for example. Thus, to ensure the charge level of the 12V or 24V low-voltage battery, the DC/DC converter must be active, at least periodically, in order to charge the 12V or 24V low-voltage battery from the high-voltage battery via the DC/DC converter. In addition, to manage the required functions of the charging system, several other components of the vehicle control system must be activated. Activation of these components likely will consume energy from the vehicle high-voltage battery, and thus may consume even more energy from the public power grid to maintain an appropriate charge level, thus reducing total efficiency of the vehicle.
Further, to maintain the charge level of the 12V or 24V low-voltage battery, whether the vehicle on-board charger is plugged into the public power grid or not, the DC/DC converter must be started on a regular basis or at a desired minimum charge level. That is, in order to not deplete the low-voltage battery, the DC/DC converter (which is a high-power DC/DC converter) must be active, at least periodically, such as during charging of the low-voltage battery from the high-voltage battery, at a minimal power level of typically below 10% of the optimal or maximum capacity. The high-power DC/DC converter typically is not designed to provide optimal efficiency at such a low power output. Further, the “idle” current drawn from the high power DC/DC converter logics, and other supporting system components required to be active, may also contribute to a higher overall energy consumption. For at least the reasons described above, the total efficiency of the vehicle may be reduced over its use cycle, and strain may be added to the DC/DC converter, thus potentially reducing its useful life.
One possible solution to the above problems is to include an additional on-board power supply for supplying the control and supervision logic before, during, and after charging, in order to maintain the charge level of the low voltage battery. As the power required to operate the additional on-board power supply can be several hundreds of watts, various requirements, such as for Power Factor Correction (PFC) etc., apply.
U.S. Patent Application Publication US 2011/0273136 to Yoshimoto, which is incorporated by reference herein, discloses an electric vehicle having a charger for charging, from an external AC power supply, a high voltage battery for driving a vehicle, and a small DC/DC converter for charging, from the high-voltage battery, an accessory low-voltage battery for driving accessories. A transformer is used for insulating a high-voltage circuit from a low-voltage circuit. US 2011/0273136 discloses a structure in which three windings (a winding for the high-voltage main battery, a winding for the AC source, and a winding for the low-voltage battery) share a single transformer, and electric power conversion is accomplished by way of a common core. However, a drawback to this approach is that it is complicated to integrate two battery chargers running at different voltages with a common core. Further, no galvanic barrier is provided between the AC main and the low-voltage battery, which is a potential safety concern, and may not satisfy existing safety regulations.
U.S. Patent Application Publication US 2010/0019723 to Ichikawa, which is incorporated by reference herein, discloses a vehicle charge system for charging high-voltage main batteries via a commercial power supply, and a low voltage power generating unit that passively generates low voltage power when coupled to the commercial power supply. In particular, during charging via the commercial power supply, the low voltage power generating unit charges a sub battery, thus enabling a controller to be operated via the sub battery. However, according to US 2010/0019723, in order for the low voltage power generating unit to charge the sub battery, the low voltage power generating unit must be connected to the commercial power supply. Further, when the commercial power supply is connected, both the sub battery and the main batteries are charged.
It would be desirable to provide a battery charging system in which a high-voltage battery is charged from an external power supply, and a low-voltage battery is charged either directly from the external power supply or from the high-voltage battery, and which reduces complexity, improves efficiency, and increases safety as compared to existing battery charging systems.